Binders which have been commonly used for magnetic recording media include thermoplastic resins such as vinyl chloride-vinyl acetate resins, vinyl chloride-vinylidene chloride resins, cellulose resins, acetal resins, urethane resins and acrylonitrilebutadiene resins used alone or in combination. However, with such binders the magnetic layer generally has poor wear resistance and the tape guiding systems which contact with magnetic tapes are stained during tape running.
Various improvements have been proposed where a thermosetting resin such as melamine resins or urea resins is used as a binder, or a binder which can be cross-linked by a chemical reaction such as an isocyanate compound or an epoxy compound is added to the abovedescribed thermoplastic resins. However, disadvantages occur when using such a cross-linking type binder. Firstly, the storage stability of the resin solution in which ferromagnetic particles are dispersed is low, i.e., the pot life is short and the physical properties of the resulting magnetic coating composition cannot be kept homogeneous, and accordingly, magnetic tapes cannot be uniformly prepared. Secondly, after coating and drying the magnetic coating composition, heat treatment is necessary to cure a coating layer thereby requiring a long period of time to manufacture magnetic recording media.
In order to eliminate the above disadvantages, a method has been proposed where a magnetic recording medium is prepared by using a combination of an acrylate oligomer and an acrylate type monomer as a binder and curing the binder by electron beam radiation after drying, as disclosed in Japanese Patent Publication No. 12423/72, Japanese Patent Application (OPI) Nos. 77433/75 and 25231/81 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), U.S. Pat. Nos. 3,871,908 and 4,368,239, German Pat. No. 2,100,037 and Dutch Pat. No. 7,118,222. However, a magnetic recording medium having satisfactory physical properties and electric properties cannot be obtained in accordance with the above-described conventional methods.
In particular, the magnetic recording medium prepared by using a binder which can be cured by electron beam radiation or a binder mixture comprising a conventional thermoplastic resin and the above electron beam curing type resin and radiating the resulting magnetic coating composition with an electron beam as disclosed in U.S. Pat. No. 4,368,239 and Japanese Patent Application (OPI) No. 25231/81 shows poor durability when the medium is used at a high temperature or the medium generates heat after repeated runs (for example, when video recording tapes are used in a still mode of video tape recorders).
Recently, it has become necessary that a support for magnetic tapes be made thinner for long-time recording and that the physical strength of the magnetic layer be improved.
Further, as video cassette recorders have become to have multi-functions, video tapes used therefor are required to withstand use under severe conditions. However, it has hitherto been impossible to produce a magnetic recording medium having physical strength which satisfies the above requirements.
The inventors have attained the present invention as a result of extensive studies for improvements of conventional methods such as a method of using thermoplastic resins and thermosetting resins as disclosed in U.S. Pat. Nos. 3,634,137, 4,238,548 and 4,307,154, a method of adding a binder cross-linkable by a chemical reaction is disclosed in U.S. Pat. Nos. 4,333,988, 4,154,895 and 4,049,871, or a method of using a curable binder cured by electron beams, as disclosed in U.S. Pat. Nos. 3,871,908, 4,004,997 and 4,343,861.